1. Field of the Invention
The present invention relates to a matrix circuit that detects a fault location in a common signal.
2. Description of the Related Art
An input apparatus using a matrix circuit is well known. As an example of such an input apparatus, a circuit for a keyboard using a general sinking matrix circuit as disclosed in Japanese Patent Application Laid-open No. S61-058025 will be described with reference to FIG. 17.
In a general sinking matrix circuit, common signal lines (XCOM) and data signal lines (XKEYD) are arranged like a lattice, with key switches arranged on these lines. An example in FIG. 17 is a sinking matrix circuit with seven common signal lines and eight data signal lines. The matrix circuit allows detection of inputs (key information) the number of which results from multiplication of the number of common signal lines by the number of data signal lines. Thus, in this circuit allows information on 56 keys to be detected. Diodes in FIG. 17 are connected together in order to prevent signals from sneaking when a plurality of key are depressed.
The key information is scanned using an ASIC or the like to drive the common signal lines in order to allow data on the data signal lines to be loaded. When a key is depressed, the common signal line to which the key is connected is driven to enable bits on the data signal line to which the key is connected, allowing the depression of the key to be recognized. For example, in FIG. 17, when a circled key switch is depressed, a common signal line XCOM2 to which the key switch is connected is driven to enable a data signal line XKEYD4. This is detected to allow the depression of the key switch at the corresponding position to be recognized.
The circuit in FIG. 17 is a sink circuit, and the common signal line is scanned by being driven low. At this time, when a data signal is low, the corresponding key switch is determined to have been depressed. Some matrix circuits are of a source type. In a sourcing matrix circuit, the common signal line is scanned by being driven high. A high input to the data signal line allows depression of the key to be recognized.
The matrix circuit allows a large amount of input information to be obtained using a smaller number of signal lines. However, a possible ground fault, short-circuit, or disconnection on the common signal lines and the data signal lines causes an erroneous input to be detected, leading to malfunction. Thus, whether a ground fault, a short-circuit, or disconnection has occurred or not and the location of the occurrence need to be determined.
When a fault occurs on the data signal lines, a phenomenon resulting from the fault and an occurrence condition for the phenomenon other than the fault on the data line are as follows.
<Ground Fault on the Data Signal Line>
                Phenomenon: all the keys connected to the data line on which a ground fault has occurred are input even when the keys are not depressed.        Failure portion on the data signal lines: the data signal line to which the input keys are connected        Cause of a fault on the common signal line leading to the phenomenon: none<Short Circuit on the Data Signal Line>        Phenomenon: depressing one key causes another short-circuited key to be input.        Failure portion on the data signal lines: the data signal line to which the two input keys are connected        Cause of a fault on the common signal line leading to the phenomenon: a short circuit on the common signal line<Disconnection of the Data Signal Line>        Phenomenon: nothing is input in spite of depression of a key.        Failure portion on the data signal lines: the data signal line to which the key that has failed to be input is connected.        Cause of a fault on the common signal line leading to the phenomenon: disconnection of the common signal line        
As described above, when a fault occurs on any data signal line, the location of the fault on the data signal lines can be relatively easily detected if the fault is determined not to have been caused by any common signal line. However, when a fault occurs on any common signal line, the location of the fault disadvantageously fails to be easily detected depending on the contents of the fault.